In positive tone (or “light field”) lithography, the area of photoresist film that is developed (removed as a result of the development process) is that which is exposed to the incident radiation. This lithographic method exemplified by chemically amplified resists mandates a radiation-induced transition in exposed areas of the photoresist that make the exposed region preferentially soluble in the development solution. The solid-phase polymer chemistry for creating preferential solvation of exposed polymer is typically carried out by (a) creating polar functional groups in the polymer through reactions such as the deprotection of a carboxylate group, as in acid-catalyzed chemically amplified resists, or (b) decreasing the photoresist polymer chain length through degradation of the photoresist polymer (e.g., e-beam lithography) or scission of selected chemical bonds in the polymer. In the case of bond scission, the new end-groups on the smaller polymers are predominantly polar.
Positive tone lithographic techniques are simpler and more common for today's aqueous base developers, and encompass an overwhelming percentage of materials in use for lithography today.
In carbon dioxide solvent systems, low-polarity polymer species such as photoresist polymers are more soluble than polar polymers. This provides an obvious pathway for negative tone image development, as seen in U.S. Pat. No. 5,665,527 to Allen and U.S. Pat. No. 6,379,874 to Ober. However, the traditional photoresist chemical transition of non-polar to polar species (which polar species are less soluble in carbon dioxide solvents) creates a contradiction and challenge in establishing a photoresist chemistry and image development technology for a positive tone lithography in carbon dioxide solvent systems. Accordingly, there is a need for new ways to carry out positive tone lithography in carbon dioxide solvent systems.